1. Field of the Invention
The present invention relates generally to a device that transmits a data stream using an allocated data slot in a distributed wireless personal area network (WPAN) and a method for the data slot allocation. More particularly, the present invention relates to a device that regulates the number of data slots to be allocated in consideration of the number of the data slots required by neighbor devices and a method for the data slot allocation.
2. Description of the Related Art
In contrast to a local area network (LAN) or a wide area network (WAN) being widely known, a personal area network (PAN) is a network owned by an individual within a range of about 10 m. Devices owned by the individual are interconnected to construct a network in order to provide convenience for the owner. A wireless personal area network (WPAN) implements an existing PAN by wireless connection.
The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.15 Working Group developed the WPAN for short distance wireless networks to standardize and implement the PAN. The IEEE 802.15 standard has four Task Groups. More particularly, IEEE 802.15.1 standardizes the well-known Bluetooth technology, whereas IEEE 802.15.3 and IEEE 802.15.3a standardize the high rate WPAN. Additionally, IEEE 802.15.4, alias ZigBee, standardizes low rate WPAN below 250 Kbps. The communication medium is shared by every device in the WPAN. In this regard, medium access control (MAC) is required to control the media access of the devices.
The MAC for the WPAN can be designed in two types of schemes: a centralized scheme and a distributed scheme. According to the centralized MAC scheme, the individual device operates for the whole network to manage and control the MAC for every device. In contrast, the distributed MAC scheme makes every device share the responsibility to manage the MAC.
FIG. 1 is a conceptual diagram of a WPAN environment according to the distributed MAC scheme. In FIG. 1, a black dot indicates a device, and a circle drawn based on the dot indicates a transmission range of a beacon from a device. According to the distributed MAC scheme, the devices share required information in cooperation with each other for the sake of channel time reservation, synchronization, and so forth. The WPAN environment according to the distributed MAC scheme (hereinafter, referred to as a distributed WPAN) adopts a concept of timing called superframe.
FIG. 2 depicts a structure of a conventional superframe.
Referring to FIG. 2, the superframe is structured as described in the Multiband Orthogonal Frequency Division Multiplexing (OFDM) Alliance draft v0.5. The superframe consists of 256 medium access slots (MAS). The superframe is 65.536 ms in length, and the MAS are 256 μs in length. Period ‘a10’ indicates a beacon period including beacon slots, and ‘a20’ is a data period including MAS used by other devices in the network to deliver a stream (data) to another device in the network. Hereinafter, the MAS in the beacon period is referred to as a beacon slot, and a MAS in the data period is referred to as a data slot. The devices can reserve and utilize a certain number of data slots.
Information as to the superframe is broadcast in the beacon slots of the beacon period allocated to a device. Neighbor devices take advantage of the superframe information received in a next superframe. A start time of the superframe depends on the beginning of the beacon period, which is defined as a beacon period start time (BPST).
In the distributed WPAN, a conventional device is assigned a certain number of data slots from the superframe as shown in FIG. 2 to satisfy a quality of service (QoS) requirement. In detail, the device transmits QoS information element (IE) via a common control channel such a beacon slot in the superframe. The QoS IE records information pertaining to a stream index designating a data stream delivered from the device, a required QoS (RMAS) requisite for the data stream delivery, and a desired QoS (DMAS) used to optimally transmit the data stream.
If the devices communicate with each other according to a same communication protocol, the modulation process, the demodulation process, and the data compression process are the same as well. Hence, the degree of QoS is proportional to the number of MAS. In this context, the reservation or allocation of QoS can imply the reservation or allocation of a certain number of MAS. Accordingly, it is possible to transmit the QoS IE with the number of required data slots and the number of desired data slots, instead of the required QoS and the desired QoS.
Upon receiving the QoS IE, the devices in the distributed WPAN record the information contained in the QoS IE received from another device in their memory. Next, the devices calculate the number of data slots or QoS to be assigned from a next superframe based on the recorded information. As the individual device requires a different number of required data slots and a different number of desired data slots, a solution is demanded to efficiently allocate the data slots.
For example, if the devices fairly distribute the data slots based on the number of devices that transmit the QoS IE, the data slots can be allocated as below.
TABLE 1Stream indexRMASDMASServiced MASA (DEV 1 → 2)20 MAS40 MAS25 MASB (DEV 1 → 2)15 MAS30 MAS25 MASC (DEV 3 → 1)40 MAS80 MAS25 MASD (DEV 4 → 3)20 MAS40 MAS25 MAS
In Table 1, when data streams A, B, C, and D are delivered in a network environment including four devices in total and the total number of MAS is 100, 25 MAS is allocated to each stream transmitted from the respective devices. RMAS of the data streams A, B, and D is 20, 15, and 20, respectively. DMAS of the data streams A, B, and D is 40, 30, and 40, respectively. As 25 MAS fairly distributed is below the DMAS but over the RMAS of each data stream, the data streams A, B, and D can be transmitted. As for the data stream C from the device 3 to the device 1, the RMAS is 40 MAS but only 25 MAS are allocated. As result, it is impossible to transmit the data stream C. As described above, under the conventional distributed WPAN, the data slots are not allocated according to the desired QoS for the provided service. Therefore, the fair allocation of the data slots cannot be attained.